1. Field of the Invention
This invention relates to a cathode and a method of fabricating cathodes for metal chloride batteries and, more particularly, of fabricating cathodes for metal chloride batteries by regulating particle sizes of the constituents in the electrode fabrication mixture.
2. Background of the Invention
The attractiveness of using electrochemical devices, such as sodium-sulfur and sodium-metal chloride (Na/MCl.sub.2) batteries, as an alternative to petroleum engines continues to increase in vogue.
The ideal battery should exhibit a number of characteristics, including low resistance and high discharge rates, operation over a wide temperature range, a capability to operate over a large number of cycles, and high energy on a volume, weight and cell basis.
Battery cells consist of two dissimilar metals in an ionically conductive medium, with the ionization potential of one metal sufficiently higher than the other metal so as to yield a voltage upon reduction/oxidation (redox) coupling above that needed to break down the electrolyte continuously at the positive electrode.
Metal typically goes into solution at the negative electrode, or anode, releasing electrons to travel in the external circuit to the positive electrode, or cathode, doing work during the transit. Material which will go through a valency drop on electrochemical discharge is included in the positive electrode. In essence, this material, the oxidizer, accepts electrons coming from the negative electrode and serves as the depolarizer. The depolarizer or cathode is positioned in the positive electrode in combination with some electron-carrying matrix, and should be porous to allow access of the electrolyte to a large area of the depolarizer. Porosity of the cathode is key as it affords greater electrode surface area and therefore a larger redox reaction surface.
The economic and social advantages of powering automobiles from batteries are considerable as the vehicles could operate at relatively high efficiencies, such as 30 percent to 40 percent, and be non-polluting. In comparison, the internal combustion engine in an automobile typically converts 10 to 15% of the energy in gasoline to motive power. Two important variables are considered in seeking an energy storage system for a vehicle. One of the variables, specific power, designated watts per kilogram (W/kg), determines to a large extent acceleration and speed capabilities. The other variable, specific energy, designated as watt-hours per kilogram (Wh/kg), determines vehicle range. The capacity density of a cell, or how much electrical energy the electrode will contain per unit volume, is designated as ampere-hours per cubic centimeter (Ah/cm.sup.3).
High power capability is theoretically achievable by use of low resistance materials and by operation at elevated temperatures which increases the charge-current density. The benefits of high temperature operation are not achieved without cost, however. For example, a large increase in solute, up to 25 percent when electrolyte salts melt, poses a design problem.
The sodium/sulfur cell has received a great deal of publicity among the high temperature rechargeable battery genre, perhaps due to its lightweight, inexpensive and compact design. This system uses the solid electrolyte beta-alumina, including .beta.-alumina, .beta.'-alumina and .beta."-alumina, that has a very high conductivity attributable to highly mobile sodium ions in cleavage planes. These cells suffer from several problems, however, including the accumulation of metallic sodium in the grain boundaries of the beta alumina, which causes shorts and also weakens the separator material. There is also a tendency of the beta alumina to lose sodium after long periods at the high operating temperatures of these cells.
Sodium/metal chloride cells are disclosed in U.S. Pat. No. 4,288,506. These cells use a sodium anode, a .beta."-alumina solid electrolyte and a MCl.sub.2 cathode with a catholyte of NaAlCl.sub.4.
Metal halide batteries exploit the higher electrolysis threshold values of electrolyte constituents to electrode constituents. During charging, the electrolyte phase becomes poorer in sodium salt with sodium metal being deposited on the anode and the halogen reacting with the reduced metal to form a metal halide. Among halides, fluorides and chlorides exhibit higher electrolysis thresholds than bromides and iodides. As such, metal chloride and metal fluoride systems exhibit relatively higher energy density and lighter mass than systems using bromides and iodides.
As with other electrochemical cells, metal halide batteries generate electricity by transporting electrons from the fuel constituent to the oxidizer, with concomitant oxidation and reduction occurring at the anode and cathode, respectively.
The following reaction occurs: EQU MX.sub.2 +2 Na.revreaction.2 NaX+M
where M is a metal such as nickel, iron, cobalt, chromium and manganese and X is a halogen such as fluorine, chlorine, bromine and iodine. The left hand side of the above equation depicts a charged state, before reduction of the metal halide, with the right hand side of the equation depicting a discharged state with reduced metal.
Utilization of the metal-chloride system is usually expressed on the basis of the ratio of the reacted NaCl to the total quantity of NaCl used to fabricate the positive electrode. This practice is convenient for the Na/MCl.sub.2 cells because they are fabricated in the discharged state and the MCl.sub.2 active material is formed electrochemically, as noted in the above cell reaction.
Despite the high theoretical specific energy of Na/MCl.sub.2 cells, specifically 790 Wh/kg for Na/NiCl.sub.2 designs, the present state-of-the-art Na/MCl.sub.2 battery exhibits specific energies of approximately 120 to 150 Wh/kg. Low specific energies may be due to an inefficient use of metal in the positive electrode. Indeed, only 25 percent to 33 percent of nickel in a NiCl.sub.2 electrode is used in the redox reaction. Na/MCl.sub.2 batteries also have exhibited limited power, of approximately 100 W/kg, probably because of the high resistivity and subsequent voltage loss of typical metal chloride electrodes.
These high resistivities and voltage losses occur due to inefficient redox reactions at the cathode. This inefficiency may be from a paucity of ion exchange between the anolyte and catholyte which often results from overcharging whereby in some of the AlCl.sub.4.sup.- catholyte is found AlCl.sub.3. A subsequent transition metal halide forms, for example, FeCl.sub.3, which attacks the beta-alumina barrier as a Lewis acid. The products of this acid-base reaction at the beta-alumina interface can block free passage of sodium ions through the solid electrolyte. Some alteration of this inefficient ion exchange can be effected by keeping AlCl.sub.3 formation to a minimum with the addition of sodium fluoride, as disclosed in U.S. Pat. No. 4,592,969.
Inhibition of sodium through the beta-alumina also can be stymied by maintaining the oxidation state of the metal as low as possible through careful regulation of sodium and aluminum ions in the initial mixture. This technique, disclosed in U.S. Pat. No. 4,546,055, strives to keep the metal halides as close to the cathode surface as possible by inhibiting their solubility in the liquid electrolyte material.
Aside from manipulating the constituents of the catholyte to improve redox efficiencies, a restructuring of the cathode itself is feasible. Prior techniques, disclosed in U.S. Pat. No. 4,288,506, incorporate carbides to improve the skeletal structure of the cathode.
Despite performance calculations indicating that specific power and specific energy greater than 500 W/kg and 200 Wh/kg can be achieved, MCl.sub.2 technology is not approaching these values partly because current cell design has mimicked older sodium/sulfur cell fabrication techniques. Metal chloride cells typically consist of a single tubular solid electrolyte made of beta alumina about 5 centimeters in diameter. Within the solid electrolyte is a thick positive electrode and NaAlCl.sub.4, acting as a secondary liquid electrolyte. The metal chloride in Cheetah cell designs is made from a mixture of sodium chloride and excess metal. This mixture is either poured into the beta alumina tube or pressed into disks. Electrodes made in this fashion are about 6 to 20 volume percent metal and have a capacity density of about 0.2 to 0.3 Ah/cm.sup.3 and are about 2 to 2.5 mm. thick. The topographies of these electrodes have pore diameters of 10 to 30 microns (.mu.m), and thick configurations which inherently exhibit low energy capacity and moderate resistivity. These cells have area specific resistance (ASI.sub.15sec) values of about 4 to 6 .OMEGA.cm.sup.2. These thick designs as well as random selection of metal and sodium chloride particle sizes is partially responsible for the lower than expected specific power and specific energy data derived from these designs.
As the metallic fraction of the electrode typically exhibits a very low resistance, the bulk of the resistance for the positive electrode must result from transfer of sodium ions through the electrolyte and diffusion at the solid surfaces in the interior of the electrode. Increasing the utilization of existing metal in cathode structures, in effect lowering the capacity ratio, will result in as yet unattainable decreases in resistance. Therefore, an alternative to the above methods to increase redox efficiencies is a systematic approach to creating larger and more uniform pores in the cathode structure. The present invention shows that electrode constituent prefabrication particle sizes have a marked effect on electrode performance. The invention fulfills a need for a method of increasing the fraction of metal actually used in the redox reaction, and therefore increasing the specific performance values of cells, through the development and fabrication of thin, large surface area, high capacity density electrodes in metal chloride cells.